Drug allergy describes clinical adverse reactions that are proved or presumed to be immunologically based. Allergic
drug reactions do not resemble
pharmacologic actions of the incriminated
drug and may occur at fractions of what would be the therapeutic dosage. Allergic
drug reactions are unpredictable; nevertheless, there is increased risk of
drug hypersensitivity in (1) patients with
cystic fibrosis who receive
antibiotics; (2) patients with human immunodeficiency virus/
acquired immunodeficiency syndrome (HIV/
AIDS) who receive
trimethoprim/sulfamethoxazole of if
HLA-B*5701(+) and receive the
antiretroviral agent,
abacavir; (3) other genetically susceptible populations such as Han-Chinese who are
HLA-B*1502(+) who develop
Stevens-Johnson syndrome and
toxic epidermal necrolysis from
carbamazepine or if
HLA-B*5801(+) are at increased risk for such reactions from
allopurinol; and (4) patients with a history of previous compatible
allergic reaction to the same medication, similar class, or potentially unrelated medication. Specific patient groups at higher risk for
drug allergy include those with Ebstein-Barr
virus infection,
chronic lymphatic leukemia, HIV/
AIDS,
cystic fibrosis, patients with
seizures being treated with
antiepileptic medications, and patients with
asthma (especially severe
asthma) who are at increased risk of
anaphylaxis from any cause including drugs compared with patients without
asthma. In patients with a history of
penicillin allergy, skin testing helps clarify the current level of risk for
anaphylaxis by using the major (
penicilloyl-polylysine) and minor
penicillin determinants where sensitivity is 99%. If
penicilloyl-polylysine and
penicillin G are used for skin testing, the sensitivity is ∼85%. When skin tests are negative, graded challenges are performed to administer optimal or truly essential
antibiotics.